Following the identification of the 4-carbon units making up the o-xylene moiety, biosynthetic studies on riboflavin will be continued with major emphasis on elucidating mechanistic aspects of the conversion of ribulose 5-phosphate into the 4-carbon unit and its further conversion into dimethylribityllumazine and riboflavin. These studies will use enzymes purified from Candida guilliermondii and Bacillus subtilis and will focus on tracing the fate of various hydrogen atoms of the precursor and establishing the steric course of formation of the methyl groups. The mechanism of the reduction of the ribosylamino to the ribitylamino linkage in procaryotes will also be examined. Further studies will focus on the stereochemistry of formation of the ribose-derived moiety of tetrahydrobiopterin, including the methyl group, and on the mode of formation of the dimethylbenzimidazole moiety of vitamin B12. The latter studies will use stable isotope labeling followed by NMR analysis of the product. Finally, the biosynthesis of 8-hydroxy-7,8-bisnor-5- deazariboflavin in Streptomyces will be investigated and the results will be compared to those in Methanobacterium thermoautotrophicum, another producer of this 5-deazaflavin.